Using nonvolatile memories in memory hierarchy has been investigated to reduce its energy consumption because nonvolatile memories consume zero leakage power in memory cells. One of the difficulties is, however, that the endurance of most nonvolatile memory technologies is much shorter than the conventional SRAM and DRAM technology. This has limited its usage to only the low levels of a memory hierarchy, e.g., disks, that is far from the CPU. In this paper, we study the use of a new type of nonvolatile memories -- the Phase Change Memory (PCM) as the main memory for a 3D stacked chip. The main challenges we face are the limited PCM endurance, longer access latencies, and higher dynamic power compared to the conventional DRAM technology. We propose techniques to extend the endurance of the PCM to an average of 13 (for MLC PCM cell) to 22 (for SLC PCM) years. We also study the design choices of implementing PCM to achieve the best tradeoff between energy and performance. Our design reduced the total energy of an already low-power DRAM main memory of the same capacity by 65%, and energy-delay 2 product by 60%. These results indicate that it is feasible to use PCM technology in place of DRAM in the main memory for better energy efficiency.

This article investigates the leader following of underactuated surface vessels that operate in the presence of environmental disturbances, model uncertainties, and actuator saturation. What makes the design of controller challenging is that the sway cannot be directly controlled. Furthermore, the actuator is formulated with magnitude saturation and rate saturation. To express the effect of magnitude saturation and rate saturation, a continuous function is employed to model actuator. An auxiliary dynamic system is used to solve the problem of magnitude saturation and rate saturation. A disturbance observer is designed to provide estimation of the environmental disturbances and model uncertainties, and achieve finite time convergence of estimation error. A control law is designed based on disturbance observer, backstepping technique, and auxiliary dynamic system. The uniform ultimate boundedness of all signals in the closed-loop control system is proved. Simulations demonstrate the performance of the proposed controller.